One technique for doping silicon wafers is to direct a beam of ions to impinge upon such a wafer to produce controlled concentrations of doping impurities within the wafer. An important parameter of a semiconductor wafer ion implant processes is the spatial uniformity of the implanted dose across the wafer. Another important parameter is the angle of incidence of the ion beam with respect to the wafer and the internal lattice structure of the silicon crystal (or GaAs lattice or other crystalline substrate). The angle of incidence is important because of the role it plays in the phenomenon of channeling. Dopant depth profiles vary as a function of position on the wafer surface if the incident angle of the beam varies across the surface.
In medium and low current ion implanters the ion beam is commonly directed across the wafer surface by x-y deflection scanning of the beam in a raster or similar pattern. This has commonly been done using two orthogonal pairs of electrostatic deflection plates or electrodes to produce the beam deflections. Application of a triangular waveform voltage to the plates can produce rectangular raster scan patterns on the wafer. U.S. Pat. No. 4,514,637 to Dykstra et al. discloses one such medium to low current ion implanter. The disclosure of Dykstra et al. patent is incorporated herein by reference.
Electrostatic ion beam deflection scanning results in different angles of beam incidence at different locations on the wafer surface. This is the source of a major depth nonuniformity (from the resulting channeling variations) which occurs in this type of implanter.
High current implanters move the wafer through a stationary beam by mechanical means such as attaching the wafers to a spinning disk passing through the beam or by other well known mechanical scanning techniques. These mechanical scanners have tended to be designed so that as the wafer moves through the ion beam, the angle of beam incidence remains constant or nearly so. For this reason mechanical scanning of wafers through a stationary beam has come to be considered as a superior method to x-y deflection of the beam for producing depth uniformity and minimum mask shadows variation.
Batch wafer handling imposed by the high current ion beam techniques have generally resulted, however, in reduced wafer throughput and required large costly wafer handling stations. Deflection scanning machines on the other hand have advantages in size, simplicity, and cost but suffer from varying angle of beam incidence problems.
To help eliminate the effects of channeling, the wafer is tilted at an angle relative to the incident beam. The wafer tilt causes the beam velocity to be lower (higher dose) for areas of the wafer closer to the source and beam velocities to be higher (lower dose) for areas of the wafer farther from the source.
Traditional deflection systems use a high frequency scan in one direction and a low frequency scan in an orthogonal direction to sweep across a circular semiconductor wafer. The relationship between the low and high scan frequencies are selected to produce a highly interlaced lissajous pattern. The scan pattern produced at the target is rectangular or square and results in scan inefficiencies because of beam portions that overscan the circular wafer. This square pattern generated by the combination of high and low frequency scan signals necessitates two visual displays for beam tuning and centering. One low frequency sweep display is in sync with the low frequency beam scan signal and one high frequency sweep display is in sync with the high frequency beam scan signal. Experience with this type of beam tuning indicates that is may not accurately center the beam on the workpiece.